1. Field of Invention
This invention relates to dynamic determinations of true mass of a body that deforms internally under acceleration and is subject to noise. More specifically still this invention relates to determining the mass of a vehicle by dynamic methods when a signal is corrupted by low frequency noise. Most specifically this invention relates to methods and apparatus installed under a fifth wheel hitch to accurately determine the weight of a semi-trailer on paved roads or agricultural fields.
2. Description of Prior Art
Weight restrictions on heavy commercial vehicles require precision weighing of vehicles, often in locations where a bridge scale is not convenient. Drive train efficiency also depends on knowledge of vehicle mass. Crop yield maps used in precision agriculture rely on near real time mass determinations during harvest operations. Finally, the quality of product may suffer without real time semi-trailer weights during field loadings. Trailers are loaded and weighed and overloaded trailers are off-loaded onto hot pavement. Fresh produce may sit in the sun for hours before being scooped up and dumped into an under loaded trailer.
All efforts at designing on-board scales fall under static or dynamic methods or some combination of those two. Static measurements have two advantages over the dynamic methods in the prior art. Internal deformation is not an issue under the constant acceleration of gravity and any noise above the single zero hertz frequency can be simply, quickly and easily low pass filtered. On the other hand all static methods require installing transducers on the frame or suspension of each wheel. See U.S. Pat. No. 5,393,936 (Tyhy, et. al.) for force transducers and U.S. Pat. No. 3,854,540 (Holmstrom) for pressure transducers in the air bag suspension. These systems are expensive, introduce reliability and maintenance issues and, in the case of combination vehicles, only work with trailers fitted with the appropriate sensors. Instead of retrofitting every semi-trailer it is desirable to install all the apparatus on-board the motive vehicle.
Previous attempts at dynamic methods of mass determination have included utilizing the resonant frequency of the drive train, U.S. Pat. No. 4,656,876, (Frend) and accelerating the vehicle, U.S. Pat. No. 7,430,491 (Gutierrez, et. al.) or decelerating, U.S. Pat. No. 6,144,928 (Leimbach, et. al.). The second two methods, in either straight trucks or combination vehicles, have required estimating or neglecting the effect of zero hertz rolling resistance and wind drag in the force or other motive signal. Since an incline would also introduce an offset to the force transducer, the determination was restricted to a level surface.
U.S. Pat. No. 6,347,269 (Hayakawa, et. al.) removes the errors introduced by zero hertz factors by eliminating the zero hertz components from both the acceleration and force signals to determine apparent mass. The method, however, is for an ideal rigid body where apparent mass equals true mass at all frequencies. Real world motor vehicles, especially semi-trailers loaded with double stacked pallets or other loosely stowed cargos that shift or deform under changes in acceleration may not be considered rigid bodies. The relative movement will cause the frequency response to deviate from that of a rigid body and the error will be too great for compliance to weight restrictions. Instead of solving that problem the Hayakawa exacerbates it by high pass filtering the signals to remove the noise resulting from low frequency changes in inclination during data gathering. High pass filtering also amplifies the high frequency noise from the interaction of surface irregularities and a vehicle suspension introducing multiple crossings errors in the rectification time averaging step. Low frequency signal bandwidth noise is a source of error but high pass filtering of the acceleration signal introduces more error than it eliminates.
The errors from resistive and spring like mechanical impedance can be greatly reduced for all vehicles currently instrumented for mass determinations just by changing the signal processing method. In the case of a fifth wheel combination vehicle fitted with a dedicated force sensing apparatus the reduction in error is enough to allow for great savings in the time and fuel wasted making trips to bridge scales.